ISO IiIFE OP isodynamic, the isoclinic, and the isogonic lines, and by imagining three lines graphically drawn over the earth., he showed thereby the vibrating and advancing direction (curves) of that mysterious force. Observa- tions of this Mnd are extremely difficult and arduous to make, and Humboldt thinks that not for centuries •will it be possible to understand the history of these intricate magnetic lines by accurate systematic obser- vation. As he has always pursued this subject with great interest., he endeavoured to institute such regular experiments. Through his exertions Europe, Asia, Africa, &c., have been covered since 1828 with a cor- responding net of magnetic observatories, extending from Toronto in Upper Canada to the Cape of Good Hope and Yan Diemen's Land—from Paris to Pekin. The discoveries of Oerstedt on electro-magnetism., and the corresponding results of Arago and Faraday were very welcome to Humboldt. Oerstedt found that electricity developed near a body being a conductor of electricity generates magnetism, while Faraday remarked that magnetism so developed would, on the contrary, also generate electric currents. Hence it follows that magnetism is one of the numerous forms in which electricity shows itself, and science acknow- ledged that the two forces were identical.^ But the question of the last named of the physical develop- ments of the many and intricate phenomena of eartli- naagnetism is not yet answered. It is yet unexplained whether the* constant change in the direction of the magnetic phenomena—which would seem to indicate various systems of electric streams in the earth—is excited directly by the unequal distribution of heat, or whether it is introduced by the solar heat, whether the planetary revolutions influence it, or whether the currents in the atmosphere have their origin in the space between the planets, in the polarity of the sun or the moon. But the magnetic observatories erected at Htimboldt's instance will assist the solving of this * Pliny liad already surmised this.